This paper presents stiffness analysis of a family of three/four degree-of-freedom pick-and-place robots with four identical limbs. The Cartesian stiffness matrix of the robots is modeled with the virtual spring approach. By means of a nondimensionalization of the stiffness matrix, the 6 × 6 inhomogeneous stiffness matrix is decomposed into two homogeneous sub-matrices, of which two performance indices, corresponding to the translational and rotational stiffnesses, are defined to evaluate the stiffness behaviors of the manipulators. A comparative study among the robot counterparts is carried out with respect to the elasto-static performance.

This paper deals with the parametric optimum design of a parallel Schoenflies-motion robot, named "Ragnar", designed for fast and flexible pick-and-place applications. The robot architecture admits a rectangular workspace, which can utilize the shop-floor space efficiently. In this work, the parametric models of the transmission quality, elasto-statics and dynamics are established. By taking into consideration of design requirements and pick-and-place trajectory, a comprehensive multi-objective optimization problem is formulated to optimize both kinematic and dynamic performances. The Pareto-front is obtained, which provides optimal solutions to the robot design. Robot prototyping work based on the optimal results is described.